HISTORY OF INSTRUMENTAL SEISMOLOGICAL OBSERVATIONS IN SWEDEN

by

Ota Kulhanek and Rutger Wahlstrom

Seismological Department, Uppsala University, Box 2101, S-750 02 Uppsala, Sweden

Abstract

Instrumental seismology in the Nordic coutries started with the installation of the Wiechert horizontal-component seismograph in Uppsala in 1904. This instrument is still in operation with practically unchanged characteristics. Other early seismograph stations in Sweden were located at Vassijaure (Wiechert 1906-1915), Abisko (Wiechert 1915-1951; Golitzin 1919-1943) and Lund (Wiechert 1917-1953). A modernization and extension of the Swedish Seismograph Station Network took place in the 1950s and 1960s with the installation of many electromagnetic seismometers. The current ( 1996) network run by the Seismological Department, Uppsala University, consists of stations at six sites, Uppsala, Kiruna, Umea, Uddeholm, Delary and Myrviken. During a one-year period, 1969-1970, Uppsala University also operated a telemetric triangular array with an aperture of 100 km and the main station in Uppsala.

Seismic monitoring has also been performed by the National Defence Research Institute (FOA). The Hagfors array, with the main task to detect and analyse underground nuclear explosions, opened in 1969. Since 1994, the basic record analysis is made at NORSAR, Norway. A mini-array was opened at Nasudden (Mala), Lappland, in 1989. In 1979-1980 a regional telemetric network of 17 stations was installed in the southern part of Sweden, with the purpose to record the local seismicity and analyse its impact for the safety of nuclear power plants and the storage of radioactive waste in the bedrock. The stations were operated until 1985 (first station closed) to 1994 (last station closed). Local telemetric networks were operated in northern Sweden 1987-1989 (six stations), and in southeastern Sweden 1987-1988 (three stations) and 1987-1993/1994 (three stations).

1 The Uppsala Wiechert seismograph

The birth of seismological instrumental observations in Sweden - and Fennoscandia - dates back to the beginning of the 20th century or, to be more specific, to October 4, 1904, when a Wiechert-type mechanical instrument, a 1000 kg inverted horizontal pendulum, started operating in the Observatory Park in Uppsala, in a building erected specially for this purpose. The installation of the seismograph was a result of a fruitful cooperation between the universities in Gottingen and Uppsala (see Schroder, 1988). Professor (in meteorology) H. H. Hildebrandsson (1838-1925) was in contact with professor E. Wiechert (1861-1928) through works on atmospherical electricity. Due to this acquaintance and also to results of the International Seismological Conference in Strassburg in 1901, Hildebrandsson tried and succeeded to establish convenient conditions for seismological observations in Uppsala. Wiechert, who in 1898 became director of the Geophysical Institute in Gottingen, started seismological observations already at the end of the last century. F. Akerblom (1869-1942), the assistant of Hildebrandsson, visited Gottingen to learn about the seismological instrumentation and seismogram interpretation. After his return to Uppsala, he prepared the

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installation of a seismograph within the premises of the Meteorological Institute. As mentioned above, the instrument was deployed in October 1904 under the supervision of G. Bartels, an engineer from Gottingen. Akerblom later became professor of meteorology and the first to carry out seismological research in Sweden.

The Uppsala Wiechert seismograph has been in continuous operation since 1904. The practically unchanged characteristics during the entire period of nine decades allow immediate comparison between earthquakes from the beginning of the century and more recent events (Kulhanek, 1988). Amplitude characteristics of the two horizontal components are shown in Figure 1. For comparison, the response curve of a Golitzin electromagnetic seismograph is also displayed in the figure. This type of instrument was installed in Abisko in Swedish Lapland in 1919 (see below). As seen in the figure, the response of electromagnetic instruments decays with f3 at very long periods, whereas the corresponding decay for mechanical systems is f2• Therefore, Golitzin historical seismograms are preferred for studies of waves in the period range 5-20 s and Wiechert records are used for waves with longer periods, e.g., mantle waves (see Okal, 1992).

1000

500

z <( 100 (!)

50

20

Figure 1

2 5 10

PERIOD(s)

20 50 100

Amplitude response curves for the Uppsala Wiechert horizantal components, W-E and W-N, and the Abisko-Kiruna Golitzin vertical component, G (prior to February 1972).

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At this writing (1996), more than 65,000 Wiechert records have been produced in Uppsala, all stored in archives at the Seismological Department. Uppsala Wiechert seismograms were systematically analysed from October 1904 to December 1955, except for the time from June 1905 to June 1906. All collected relevant information is summarized in annual Seismological Bulletins from Uppsala. The Bulletin provides the date, phase identification(s) and corresponding arrival time(s) for each recorded event. For a large event, the period(s) and amplitude(s), epicental distance, focal region and often a brief commentary are also given. Exceptionally (more recent events), information on the focal depth and first-motion polarity is included. The Bulletin also includes calibration constants, which were determined annually or more frequently. After 1959, the regular calibration tests were discontinued, leaving some uncertainty as to the instrumental response. However, frequent parallel magnitude determinations from Wiechert and other seismograms indicate that the response characteristics are rather stable *J.

Uppsala Wiechert records constitute one of the most extensive, almost uninterrupted, homogeneous and well documented seismogram series available in the world, this in respect to the long time of operation, practically unchanged instrument constants and regular record analysis. This and similar series are invaluable for studying earthquake history of various regions and for comparative investigations of large historical earthquakes. The Seismological Department obtains frequent requests for record copies from observatories around the world.

2 Other seismograph stations before 1950

Several other seismograph stations operated in Sweden early in this century. In 1906, an 80 kg vertical-component Wiechert seismograph was deployed at Vassijaure in northern Sweden. In 1915, the apparatus was moved toAbisko where its operation ceased in 1951. In 1919-1943, the Abisko station was also equipped with a three-component Golitzin system with photographic recording via mirror galvanometers.

During the period 1917-1953, a Wiechert seismograph similar to that in Uppsala was in operation in Lund in southern Sweden. From 1917, there were thus three seismograph stations operating in Sweden, covering the country relatively well in the north-south direction. However, in contrast to Uppsala Wiechert records, seismograms from Vassijaure/Abisko were not analysed in a regular way and the available record collection, stored in Uppsala, is far from complete. The Lund records were analysed and stored in Copenhagen, from where they were transferred to the Uppsala archive in the 1980s. This series of records is also incomplete. Details of the operation of historical (first half of this century) Swedish seismograph stations are given in Table 1 and station locations are shown in Figure 2.

3 Uppsala University operated stations

3.1 1950s and 1960s: Formation of a national network

A vast instrumental modernization and station densification took place in Sweden in the 1950s and 1960s on the iniative of M. Bath (professor in seismology 1967-1981). Altogether, six

*) A calibration in December 1996 showed that no significant change has taken place.

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permanent seismograph stations, all with recording on photographic paper, were operated by the late 1960s. Detailed information about all Uppsala University operated stations after 1950 is summarized in Tables 2 and 3 and the station locations are plotted in Figure 2. This network is named the Swedish Seismograph Station Network, in short SSSN.

In Uppsala, in 1950-1951, a new room with three large concrete piers was added to the old seismograph house and photographic recording rooms were arranged. A Grenet-Coulomb vertical-component seismograph was installed in 1951 and Benioff three-component instruments in 1955. In collaboration with Lamont-Doherty Geological Observatory, Columbia University, New York, three Press-Ewing instruments were installed and started operating in 1957.

In Kiruna in Swedish Lapland, a seismograph building was erected in 1951 on the premises of the Geophysical Observatory of the Royal Swedish Academy of Sciences. A Grenet­Coulomb instrument, similar to that in Uppsala, was installed in 1951. Three Golitzin instruments from the old station at Abisko were at the same time repaired and re-installed at Kiruna. In 1963, a vertical-component Press-Ewing instrument was also deployed.

After a thorough search for a suitable location in the province of Jamtland, a seismograph house was built in 1955 on the property belonging to the farm of Skalstugan. The instrumental installation (Grenet-Coulomb vertical-component) was finished by the end of 1955 and the continuous operation started in January 1956.

First plans· for a seismograph station in Goteborg were made already in 1954. After searching for a suitable recording site, the station was installed in the basement room of the Physics Department of Chalmers Institute of Technology. Following a period of test recordings, continuous operation started in January 1958. The instrument was the same vertical-component Grenet-Coulomb previously operated in Uppsala. The operation of the Goteborg station was discontinued in 1968.

The Umea station, located near the coast of the Gulf of Bothnia, operated a vertical­component Grenet-Coulomb seismograph from 1960 to 1962. In 1962, the world-wide standardized equipment of the U.S. Coast and Geodetic Survey was installed. Umea was a complete WWSSN station with three-component, short-period (Benioff) and long-period (Press-Ewing) instruments.

The station Karlskrona on the south coast of Sweden was also provided with a vertical­component Grenet-Coulomb instrument, similar to that in Uppsala. This station was opened in 1961 and closed down in 1968.

The most sensitive station in the Swedish network is Uddeholm in the province of Varmland. During the period 1966-1967, U ddeholm operated a vertical-component Grenet-Coulomb instrument. In 1967, this was replaced by a Benioff vertical-component seismograph.

As·mentioned above, the stations at Goteborg and Karlskrona ceased their operations in 1968. The reason was that a more sensitive site had been found in south Sweden, near Delary in the province of Smaland. Delary operates a vertical-component Grenet-Coulomb seismograph since 1967.

Beside these stations, sensitivity tests were made at several other sites in the years 1964-1966,

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using Grenet-Coulomb seismographs. Temporary station sites were located at Askersund (province of Narke), Bollnas (Halsingland), Haparanda (Norrbotten), Hermanstorp (Smaland), Lycksele (Lappland), Mora (Dalama), Stromsund (Jamtland), Suppovaare (Lappland), Sveg (Harjedalen) and U ggelheden (Varmland).

3.2 The Uppsala triangular array

During a one-year period, from August 1969 to August 1970, the Seismological Institute, Uppsala operated a simple triangular array. The triangle was almost equilateral with an aperture of around 100 km and the sensors at the permanent station in Uppsala, Hedemora and Kungsor (Tables 2 and 3; Figure 2). All three stations were equipped with Grenet-Coulomb vertical-component seismographs. The signals were continuously transmitted over commercial telephone lines to the central recording unit in Uppsala (Borg and Bath, 1971). Extremely favourable geological conditions and consequently high signal coherence across the array (Kulhanek, 1973) prove that the area around Uppsala very well serves the purpose of array­station recording.

3.3 1980s: Modernization at Uppsala

Further modernization of SSSN took place in the 1980s, mainly at the central station in Uppsala. In 1983, a three-component, short-period system equipped with Teledyne-Geotech S-13 sensors and hot-stylus recording was installed. In 1989, a three-component broadband system equipped with leaf-spring Wielandt-Streckeisen seismometers was .deployed together with a data acquisition system with 16 bits resolution and 20 Hz sampling, employing an IBM/ AT personal computer.

3.4 Current SSSN

The current SSSN consists of six permanent stations, Uppsala (UPP), Kiruna (KIR), Umea (UME), Uddeholm (UDD), Delary (DEL) and Myrviken (MYV), a configuration that almost equally covers the whole territory of Sweden (Figure 2). The stations operate a varying number of seismographs and altogether 24 channels are now in operation (Table 3).

The station at Myrviken in the province of Jamtland started its operation in December 1981 as a substitute for Skalstugan, which was discontinued in September 1981 due to logistic difficulties. Myrviken is equipped with an S-500 Teledyne-Geotech seismometer and a Portacorder with ink recording.

In Kiruna, the two medium-period, horizonal-component Golitzin seismographs and the ultra­long period, vertical-component Press-Ewing seismograph were closed down in 1982. In Umea, the operation of the long-period instruments was discontinued between December 1990 and May 1994, but today the station is running just like in the WWSSN time.

A quartz clock is used for the timing at each of the six stations. Daily comparison with the time signal issued by the Swedish radio was used (except for MYV) until December 1993. Since December 1993, stations UPP, UME, UDD and DEL are making use of the DCF Mainflingen, Germany, transmitter. MYV is using the DCF signal since the time of installation

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in 1981 and KIR employed a OPS system from December 1993 to October 1994 and an OMEGA system after this period. The UPP S-13 recording is equipped with a standard Teledyne-Geotech timing system, model TG-120, with a 1.536 MHz temperature-controlled crystal oscillator.

Seismogram readings from the UPP, KIR and UME records are done regularly. For regional earthquakes, also records from the other stations are analysed. Larger events are reported immediately to the news media. Seismogram readings are sent regularly to international seismological centres and, as weekly (UPP only) and monthly bulletins, to observatories all over the world.

Considerable efforts have been made to establish a convenient seismogram storage system in Uppsala. After decades of improvisation, all records are now deposited in an archive at the Seismological Department. They are easily accessible for inspection and/or further analysis or processing. Upon request, we provide microfilm copies of photographic records and photographic copies of smoked-paper Wiechert seismograms.

4 Seismological monitoring by the National Defence Research Institute, Stockholm

Besides Uppsala University, seismological monitoring and research in Sweden have been performed by The National Defence Research Institute (FOA) in Stockholm. The main task for FOA in seismology has been to record and analyse underground nuclear explosions to provide seismological criteria for discrimination between this type of events and earthquakes. Monitoring of regional seismicity has been another task, specially in the early 1980s. Today, the activities in both detection and especially regional seismology are low.

4.1 The Hagfors Observatory and other stations monitoring nuclear explosions

Field measurements by FOA in preparation for the establishment of a seismic array for detection of underground nuclear explosions started in 1966 and the following year a place, Gunnerudssatem, near Hagfors in the province of Viirmland was selected as the main station. The array, including a subarray at Hagfors and single substations at Appelbo and Stollet, opened in 1969. Radio link transmitted signals from the substations were recorded, together with signals from the main station, in both analog and digital forms at Hagfors. By 1973, subarrays were installed at all three sites. The station Torsby was added in 1975. The locations of these and other FOA operated stations are shown in Figure 3. More detail is given in Table 4.

Originally, record analysis was performed both in Hagfors and Stockholm. In the early 1980s, all analysis was moved to Stockholm. The Hagfors Observatory was later modernized to serve as a potential prototype station for monitoring underground nuclear explosions (for technical details, see Seismology 1989-1992, 1993). Since 1994, the basic data analysis is performed at NORSAR, Kjeller, Norway and readings are included in their bulletin.

In 1989, after more than a year of testing, a mini-array started operating in Nasudden in northern Sweden. The site was chosen to be close to the centre of the existing Fennoscandian arrays - ARCESS, NORESS, FINESA and Hagfors (see the Norwegian and Finnish contributions of these Proceedings).

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4.2 Networks for monitoring regional and local seismicity

Starting in 1979 (15 stations) and 1980 (two stations), a dense regional network of short­period seismographs was installed in the southern part of Sweden, up to approximately lat. 61°N (Figure 3). Together with the Hagfors array and three similar stations in Denmark, the result was a significant improvement in event detection and other signal analysis. Three of the Swedish and one of the Danish stations had three-component sensors and the remaining stations had a vertical-component sensor. The digital signals were transmitted on permanent telephone lines to a computer centre at FOA.

The Swedish part of the project was financed by the Swedish Nuclear Power Inspectorate (SKI) with the purpose to get a detailed picture of rock movements and stress patterns in the bedrock connected to earthquake activity, information of use for the seismic safety of nuclear power plants and potential storage of the radioactive waste in the bedrock. The project terminated in 1984/1985 and stations started closing down from 1985. The longest operating stations remained open until late 1994 (see Table 4).

Connected to projects funded by the Swedish Nuclear Fuel and Waste Management Co (SKB), two six-station, short-period, vertical-component local networks were operated from the autumn of 1987 (Figure 3). One was located in northern Sweden and closed down in the spring of 1989. The other was located in southeastern Sweden, with three stations closing in the late 1988 and the remaining three in 1993/94. Four of the stations of the southern network are on sites identical to SKI station locations. Again, records were transmitted on permanent telephone lines to the computer centre at FOA in Stockholm.

Acknowledgements

To build up, maintain and operate the SSSN would not have been possible without the essential economic contributions from the Swedish Natural Science Research Council, Ministry of Education, Uppsala University, Swedish Nuclear Fuel and Waste Management Co, National Board for Spent Nuclear Fuel, Knut and Alice Wallenberg Foundation, Royal Swedish Academy of Sciences, City of Kiruna and Skalstugan Foundation. Hopefully, future extended funding will facilitate a much needed upgrade of the SSSN (more broadband stations, digital recording, telemetric data transmission, etc.). L. Nordgren contributed material to the section on FOA's activities.

References

Borg, H. and M. Bath (1971): The Uppsala seismograph array station. Pure Appl. Geophys., 89, 19-31.

Kulhanek, 0. (1973): Signal and noise coherence determination for the Uppsala seismograph array station. Pure Appl. Geophys., 109, 1653-1671.

Kulhanek, 0. (1988): The status, importance, and use of historical seismograms in Sweden. In: W.H.K. Lee, H. Myers and K. Shimazaki (Eds), Historical Seismograms and Earthquakes in the World. Academic Press, pp. 64-69.

Okal, E. (1992): Use of the mantle magnitude Mm for the reassessment of the moment of

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historical earthquakes. Pure Appl. Geophys., 139, 17-57.

Schroder, W. (1988): Uppsala und Gottingen: Streiflichter zum Beginn der internationalen seismologischen Kooperation. Gerlands Beitr. Geophys., 97, 450-459.

Seismology 1989-1992 (1993): Nuclear Test Ban Verification. FOA Report C20926-9.l (2.2), 57 pp.

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TABLE 1. List of Historical Seismograph Stations in Sweden

Station Lat. Long. Seismograph Record Time of (oN) (oE) type, comp. paper operation

Uppsala 59.86 17.63 Wiechert E,N smoked 1904-

Vassijaure 68.42 18.18 Wiechert Z smoked 1906-1915

Abisko 68.34 18.82 Wiechert Z smoked 1915-1951 Golitzin E,N,Z photo 1919-1943

Lund 55.70 13.19 Wiechert E,N smoked 1917-1953

TABLE 2. Site Information of the Swedish Seismograph Station Network and the Auxiliary Array Stations at Hedemora and Kungsor

Station Code Lat. Long. Altitude Geology 1) (oN) (oE) (m)

Uppsala UPP 59.858 17.627 14 granite

Kiruna KIR 67.840 20.417 390 porphyry

Skalstugan SKA 63.580 12.280 580 gneiss

Goteborg Gb 57.698 11.978 66 gneiss

Umea UME 63.815 20.237 16 mica gneiss and pegmatite

Karlskrona Ka 56.165 15.592 11 granite

Uddeholm UDD 60.090 13.607 240 granite

Delary DEL 56.470 13.870 150 gneiss

Myrviken MYV 62.942 14.347 345 granite

Hedemora Hd 60.288 15.948 124 gneiss-granite

Kungsor Ku 59.402 16.133 63 gneiss

1) Two-letter codes are unofficial.

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TABLE 3. Instrumentation of the Swedish Seismograph Station Network and the Auxiliary Array Stations at Hedemora and Kungsor; Full Stations Names are given in Table 2

Code Seismometer To Tg Max. Recording Time of type, component (s) (s) magnif. type, paper operation

UPP Wiechert E 11 240 smoked 1904-Wiechert N 10 230 smoked 1904-Grenet-Coulomb Z 1.4 0.7 13,510 photo 1951-1957

& 1965 & 1969-1981

Benioff E 0.9 0.7 80,000 photo 1955-1983 Benioff N 0.8 0.7 80,000 photo 1955-1983 Benioff Z 1.0 0.7 40,000 photo 1955-Benioff E 0.9 83 3,030 photo 1955-1982 Benioff N 0.8 92 3,290 photo 1955-1982 Benioff Z 1.0 88 3,670 photo 1955-Press-Ewing E 15 103 2,270 photo 1957-Press-Ewing N 15 98 2,250 photo 1957-Press-Ewing Z 15 100 1,770 photo 1957-Press-Ewing Z 15 100 1,200 ink 1963-1983 Geotech-Teledyne 1.0 110,000 hot stylus 1983~

S-13 E,N Geotech-Teledyne 1.0 92,000 hot stylus 1983-S-13 Z Wielandt- 20 digital & 1989-Streckeisen E,N,Z hot stylus

KIR Grenet-Coulomb Z 1.4 0.7 13,310 photo 1951.-Golitzin E 12 12 770 photo 1951-1982 Golitzin N 12 12 800 photo 1951-1982 Golitzin Z 10 10 860 photo 1951-Press-Ewing Z 15 100 1,200 ink 1963-1971 Press-Ewing Z 15 100 3,800 photo 1971-1982

SKA Grenet-Coulomb Z 1.3 0.8 10,530 photo 1956-1981

Gb Grenet-Coulomb Z 1.4 0.5 12,040 photo 1958-1968

UME Grenet-Coulomb Z photo 1960-1962 Benioff E,N ,Z 1.0 0.7 75,000 photo 1962-Press-Ewing E,N ,Z 15 100 5,500 photo 1962-1990

& 1994-

Ka Grenet-Coulomb Z 1.5 0.7 11,590 photo 1961-1968

UDD Grenet-Coulomb Z 1.4 0.7 (12,990) photo 1966-1967 Benioff Z 1.0 0.7 75,000 photo 1967-

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TABLE 3. (cont).

Code Seismometer To Tg Max. Recording Time of type, component (s) (s) magnif. type, paper operation

DEL Grenet-Coulomb Z 1.4 0.7 12,990 photo 1967-

MYV Geotech-Teledyne 1.0 varying ink 1981-S-500 Z

Hd Grenet-Coulomb Z 1.4 0.7 (15,000) photo 1969-1970

Ku Grenet-Coulomb Z 1.4 0.7 (15,000) photo 1969-1970

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TABLE 4. List of FOA Operated Seismograph Stations in Sweden

Name Code Lat. Long. Time of Network(s)/ (oN) (oE) operation Instrument( s)

Abborrasen ABB 57.845 12.786 1979-1985 SKI/SPZ

Alsterbro ALR 56.995 15.918 1979-1993 SKI,SO/SPZ

Appelbo APO 60.593 13.928 1969- HFS/SP3,LPZ,4sS

Basthult BHU 57.428 16.559 1987-1988 SO!SPZ

Broaryd BYD 57.058 13.164 1980-1985 SKI/SP3

Granbergsdal GBD 59.485 14.553 1979-1988 SKI/SPZ

Hagfors HFS 60.134 13.697 1969- HFS/USPZ,SP3, LP3,BBZ,3moreSPZ,4sS

Hakkasbyn HAK 66.925 21.560 1987-1989 NO/SPZ

Hallstavik HVK 60.009 18.567 1979-1986 SKI/SPZ

Hedekas HED 58.680 11.788 1979-1985 SKI/SPZ

Horndal HRN 60.250 16.489 1979-1993 SKI/SPZ

Kalix KLX 66.067 23.031 1987-1989 NO/SPZ

Katrineberg KTB 61.067 16.283 1979-1994 SKI/SPZ

Korpilombolo KPM 66.755 22.905 1987-1989 NO/SPZ

Lansjarv LJV 66.655 22.182 1987-1989 NO/SPZ

Mariestad MRD 58.617 13.747 1980-1985 SKI/SP3

Masugnsbyn MUG 67.462 22.045 1987-1989 NO/SPZ

Nasudden NSD 65.196 18.823 1989- M-AR/SP3,4sS

Nykoping NYK 58.925 17.092 1979-1994 SKI,SO/SPZ

Olofstrom OLO 56.308 14.470 1979-1986 SKI/SPZ

Orkelljunga ORK 56.268 13.262 1979-1986 SKI/SPZ

Stang a SGA 57.303 18.472 1979-1988 SKI,SO/SPZ

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TABLE 4. (cont).

Name Code Lat. Long. Time of Network(s)/ (oN) (oE) operation Instrument( s)

St5llet SLL 60.475 13.319 1969- HFS/SP3,LPZ,4sS

Tenhult TNU 57.639 14.269 1979-1985 SKI/SP3

Torsby TBY 60.083 12.833 1975-1992 HFS/SPZ

Varmlands Ny- VNY 59.403 12.483 1979-1988 SKI/SPZ sater

Vastmark VMK 65.680 21.587 1987-1989 NO/SPZ

Veberod VEB 55.599 13.516 1979-1986 SKI/SPZ

Vimmerby VIM 57.788 12.483 1979-1994 SKI,SO/SPZ

Virserum VUM 57.328 15.488 1987-1988 SO/SPZ

Networks HFS Hagfors array or subarray within Hagfors array /samling rates USP 120 Hz,

SP 40 Hz, LP 1 Hz/ Mini-array M-AR

NO SKI so

Northern Sweden local network /sampling rate 60 Hz/ SKI network /sampling rate 60 Hz/ Southeastem Sweden local network /sampling rate 60 Hz/

Instruments SP3 short-period, 3 components SPZ short-period, vertical component USPZ ultra-short-period, 3 components LP3 long-period, 3 components LPZ long-period, vertical component BBZ broadband, vertical component 4sS 4 substations

IIlstrumentation of mini-array and subarrays are given at the peak of operation in the early 1990s.

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